The present invention relates to a safety switching device for fail-safely switching on and off an electrical load. The present invention in particular relates to a safety switching device comprising a fail-safe control unit, a first and second electronic switching element, a first and second output terminal and at least one input terminal for receiving a switching signal that causes switching off said switching elements.
The invention further relates to a safety switching device system comprising at least two safety switching devices which interact in a fail-safe manner.
Safety switching devices are, for instance used in industrial production environments in order to reliably and safely disconnect machines, such as a hydraulic press or a milling machine, if this is necessary for the safety of personnel or machinery. Safety switching devices typically have one or more input terminals to which input devices such as emergency stop buttons, two-hand operator buttons, guard door switches or light curtains can be connected. An evaluation and control unit evaluates the input signal from these input devices in a fail-safe manner and produces a switching signal on an output side being a function of same. The evaluation and control unit is also designed to operate in a fail-safe manner, for instance by providing redundancy. The switching signal controls actuators, in particular contactors for disconnecting the machine from a power supply.
The term “fail-safe” signifies that disconnecting the machine must be ensured under all circumstances, even if a functional fault occurs within the chain comprising the input device, the safety switching device and the actuator. Safety switching devices must therefore be designed such that they ensure that the machine being monitored is always in a safe state, even in the event of the failure of one component, in the case of broken cables or other malfunctions. Accordingly, the term “safety-switching device” in this context refers only to those devices and appliances which comply at least with category 3 of European standard EN 954-1, or comparable requirements.
In DE 100 11 211 B1, a safety switching device and a system of more than one safety switching device is shown, wherein the safety switching device system comprises two safety switching devices which are connected to each other via two lines. By providing two signals which are clocked in a way that they have different signals, a cross connection between those two lines can be detected by means of an additional feedback signal. With this configuration, a category 4 safety level can be achieved.
This known solution, however, suffers from the drawback that in order to be able to classify the system as safety category 4, an enhanced expenditure in terminals and additional lines has to be provided.
There is therefore a need to provide a safety switching device and a system of safety switching devices which allow for a cross-fault detection on the one hand and can be realized in a particularly simple and economic way on the other hand.